There are a large number of sound sources in an automobile. From the viewpoint of the demand for quietness away from automotive inside and outside noises, various sound-proof measures have been taken. In particular, with regard to the components (peculiar noise sources) that generate loud sounds, such as engines, transmissions and driving systems, sound-proof measures are required in the positions near to the sound sources. Thus, a dedicated sound-proof cover excellent in sound-absorbing and insulating performance is used. Combined with the tightening of automotive outside noise level regulations due to a series of legal changes and the fact that a reduction in automotive inside noise is directly linked to a car value (a touch of class), the demand for noise-reducing components in automobiles is very high. In particular, an automotive outside noise regulation scheduled to be introduced in the European Union in 2013 is finally as severe as −0.3 dB to the conventional regulation value (it is necessary to be reduced to one half in terms of sound pressure energy). This essentially requires noise reduction measures against the peculiar noise sources such as basic engines and transmissions as main noise emitting sources in an engine room. Although various sound-proof components such as engine top covers on the side of upper surfaces of engines have hitherto been used, however, further improvement in performance has been demanded. Further, from the viewpoint of a decrease in fuel consumption, weight saving have also been demanded.
Conventional sound-proof covers are designed with putting the principal objective thereof to insulation of direct noise emitted from the peculiar noise sources, and have structures in which a sound-absorbing material is post-attached to the peculiar noise source side of a rigid cover or to a part thereof, which is formed by molding a metal or a resin such as polyamide or polypropylene (see Patent Document 1). However, the sound-insulating performance of such a sound-proof cover conforms to the mass law, and depends on the weight of the rigid cover. It is therefore impossible to comply with the needs for weight saving. Further, in the case where the peculiar noise source is accompanied by vibration, even when the vibration is transmitted from fixing points and the like for attaching the sound-proof cover to the engine and the like, the rigid cover hardly undergoes vibration-induced deformation, and hence an effect of damping the vibration as kinetic energy cannot be obtained. Accordingly, secondary emission occurs from a rigid noise insulating layer to rather deteriorate the noise level in some cases.
Moreover, regarding the evaluation of noises inside and outside an automobile, since the noise level itself is an amount of sense of human, a sound pressure level (dB) obtained by logarithmically compressing an observed sound pressure is used as a criterion close to an amount of the sound sensed by human. However, when a four (multi)-directional average (combination sound) which is generally employed in a case of evaluating a general sound-proofing effect (the increase or decrease in sound pressure level) is considered, the largest sound of all the measured sounds exerts a large influence because of the characteristic of the dB sum calculation. Therefore, even though the sound pressure level in only one direction in which a sound-proof measure has been taken is reduced, the sound-proof effect could not be attained as a whole with the result that the sound pressure level that is an amount of human sense could not be lowered in some cases. Accordingly, it is necessary to thoroughly and uniformly reduce the sound pressure level in every direction.
However, with the sound-proof cover disclosed in Patent Document 1 having a structure in which a sound-absorbing material is attached to a rigid cover, the rigid cover may be resonant with vibration transmission (solid-borne sounds) in case where the peculiar noise sources is accompanied by vibration, thereby generating noises by itself, that is, causing secondary emission. In general, therefore, it is necessary to be fixed to the peculiar noise sources via a vibration-insulating material such as rubber bush. Therefore, a gap is necessarily formed between the peripheral edge of the sound-proof cover and the peculiar noise source, and there may be a case where inner reverberating sounds (standing waves) leak out from this portion and the sound level reduction cannot be attained.
From such a background, for the purpose of taking measures against solid-borne sounds in the case where the peculiar noise sources is accompanied by vibration or inner reverberating sounds (standing waves) of a sound-proof cover, the present inventors has proposed a sound-proof cover, in which a soft sound-insulating layer formed of a nonwoven fabric coated with a vibration-damping resin is provided, in place of a rigid cover, on a surface of a sound-absorbing material on the opposite side of an peculiar noise source (see Patent Document 2).
However, the sound-proof cover described in Patent Document 2 has a limitation in its mass from a manufacturing problem of the soft sound-insulating layer, and is inferior in sound-insulating performance in a high-frequency region of 4 kHz or more to a high-mass rigid cover in some cases.